The Retro Adapter has two connectors, a 9 pin and a 15 pin "D-type". You can plug Atari / Commodore style joysticks, Sega Master System / System III, and Sega Megadrive / Genesis (3 and 6 button) pads in to the 9 pin connector. Neo Geo joysticks / pads can be plugged directly in to the 15 pin connector.
For other systems all you need is the right connector, or you can modify the controller itself while still retaining compatibility with the original system.
Once plugged in, the Retro Adapter works just like any normal USB gamepad or joystick. No need for drivers or anything like that. It will work with any game or emulator, and is compatible with Windows, Mac OS X, Linux and the Playstation 3.